Semiconductor wafers are generally produced by a procedure in which a mono- or polycrystalline crystal composed of semiconductor material and having a longitudinal axis and a cross section is sliced into a multiplicity of semiconductor wafers simultaneously in one work operation with the aid of a wire saw.
The workpiece can be a cylindrical single crystal composed of silicon, for example.
The term “cylindrical” should not be understood to mean that the crystals must necessarily have a circular cross section. Rather, the crystals can have the shape of any general cylinder. A general cylinder is a body which is bounded by a cylindrical surface with a closed directrix and two parallel planes, the base surfaces of the cylinder.
Such methods are therefore also suitable for sawing non-cylindrical crystal blocks which comprise a peripheral surface, that is to say e.g. crystal blocks which have a square or rectangular cross section.
Wire saws are used, in particular, for slicing a multiplicity of semiconductor wafers, solar wafers and other crystal wafers from a crystal in one work operation.
U.S. Pat. No. 5,771,876 describes the functional principle of a wire saw suitable for slicing semiconductor wafers from a crystal.
DE 10 2006 058 823 A1, DE 10 2006 058 819 A1 and DE 10 2006 044 366 A1 describe corresponding methods for wire sawing.
Wire saws have a wire gang formed by a sawing wire wound around two or more wire guide rolls.
The sawing wire can be coated with an abrasive coating. When using wire saws having a sawing wire without fixedly bonded abrasive grain, abrasive grain is supplied in the form of a slurry during the slicing process.
In the course of the slicing process, the workpiece penetrates through the wire gang, in which the sawing wire is arranged in the form of wire sections lying parallel alongside one another. The penetration of the wire gang is brought about by means of an advancing device that guides the workpiece toward the wire gang or the wire gang toward the workpiece.
When slicing semiconductor wafers from a crystal, it is customary for the crystal to be connected to a sawing strip, into which the sawing wire cuts at the end of the method. The sawing strip is a graphite strip, for example, which is adhesively bonded or cemented on the peripheral surface of the crystal. The workpiece with the sawing strip is then cemented on a support body. After slicing, the resulting semiconductor wafers remain fixed like the teeth of a comb on the sawing strip and can thus be removed from the wire saw. The residual sawing strip is subsequently detached from semiconductor wafers.
In the case of conventional methods, sliced semiconductor wafers often have increased warp values.
It has been assumed heretofore that the parameters bow and warp as a measure of the deviation of the actual wafer shape from the ideal wafer shape sought (also “sori”) depend very crucially on the straightness of the cut. The parameter “warp” is defined in SEMI-standard M1-1105. The measurement variable warp is a measure of the deviation from an ideal wafer shape characterized by flat and plane-parallel wafer sides.
The warp also arises as a result of a relative movement of the sawing wire sections with respect to the workpiece which takes place in the course of the sawing process in an axial direction relative to the workpiece. This relative movement may be caused for example by cutting forces occurring during sawing, axial displacements of the wire guide rolls as a result of thermal expansion, by instances of bearing play or by the thermal expansion of the workpiece.
DE 10122628 describes a method for slicing a rod- or block-type workpiece by means of a saw, wherein the temperature of the workpiece is measured during slicing and the measurement signal is forwarded to a control unit, which generates a control signal used for controlling the workpiece temperature.
Furthermore, endeavors have been made to improve the guidance of the sawing wire.
DE 10 2007 019 566 A1 describes, for example, a wire guide roll for use in wire saws for simultaneously slicing a multiplicity of wafers from a cylindrical workpiece, which roll is provided with a coating having a thickness of at least 2 mm and at most 7.5 mm and composed of a material having a hardness according to Shore A of at least 60 and at most 99, which roll furthermore comprises a multiplicity of grooves through which the sawing wire is guided, wherein the grooves each have a curved groove base having a radius R of curvature equal to 0.25-1.6 times a sawing wire diameter D, and an aperture angle a of 60-130°.
The use of such a wire saw leads to an improvement in the waviness.
Besides the thickness variation, the flatness of the two surfaces of the semiconductor wafer is of great importance. After a wire saw has been used to slice a semiconductor single crystal, for example a silicon single crystal, the wafers thereby produced have a wavy surface. This waviness can be partly or completely removed in the subsequent steps, e.g. grinding or lapping, depending on the wavelength and amplitude of the waviness and also on the depth of the material removal. In the worst case, such surface irregularities (“undulations”, “waviness”), which may have periodicities of from a few mm up to e.g. 50 mm, may still be detected even after polishing on the finished semiconductor wafer, where they have an adverse effect on the local geometry.
DE 10 2006 050 330 A1 describes a method for simultaneously slicing at least two cylindrical workpieces into a multiplicity of wafers by means of a wire gang saw having a specific gang length, wherein the at least two workpieces are fixed successively in the longitudinal direction on a mounting plate, wherein a defined distance is respectively maintained between the workpieces, the latter are clamped in the wire gang saw and sliced by means of the wire gang saw.
If a low warp value of the wafers is desired, workpieces that are as long as possible are chosen. In order to achieve high warp values, comparatively short workpieces are fixed on the mounting plate and correspondingly sawn.
It has been found, however, that, despite all measures, wafers having increased warp values repeatedly occur in the prior art. This evidently cannot always be attributed to the sawing process per se or to the thermal properties of workpiece, wire guide roll, etc.